Fix “slow mode” issues (#11859)

* Fix weird scroll-jumping behavior with pending items

* Treat pending items as unread items

* Fix scroll position being altered because of the “X new items” button

app/javascript/mastodon/features/notifications/index.js

@@ -39,7 +39,7 @@ const mapStateToProps = state => ({
   showFilterBar: state.getIn(['settings', 'notifications', 'quickFilter', 'show']),
   notifications: getNotifications(state),
   isLoading: state.getIn(['notifications', 'isLoading'], true),
-  isUnread: state.getIn(['notifications', 'unread']) > 0,
+  isUnread: state.getIn(['notifications', 'unread']) > 0 || state.getIn(['notifications', 'pendingItems']).size > 0,
   hasMore: state.getIn(['notifications', 'hasMore']),
   numPending: state.getIn(['notifications', 'pendingItems'], ImmutableList()).size,
 });